Hormonal Regulation of Lipogenesis and Lipogenic Enzymes in Primary Cultured Rat Hepatocytes

Atsushi Sagayama, Tetsuo Mura*, Shiro Ikawa† and Hironaka Kawasaki

Second Department of Internal Medicine, *Department of Pathological Biochemistry and †Department of Clinicla Laboratory Medicine, Faculty of Medicine, Tottori University, Yonago 683, Japan

The hormonal induction of lipogenic enzyme activities, glucose-6- phosphate dehydroge-nase (G6PDH) [EC 1.1.1.49] and malic enzyme [EC 1.1.1.37] were studied in primary cultures of rat hepatocytes. Insulin (10^-8 M) induced G6PDH several fold in 48 h-culture. It also induced lipogenesis, measured as [1-¹⁴C]acetate incorporation in 2 h, in the cultured hepatocytes. Of the various lipids, triacylglycerol and phospholipids were induced markedly, while cholesterol and its esters were not. The increase of lipogenic enzyme activities and lipogenesis were parallel. Glucagon dibutyryl cyclic AMP (DB-cAMP), triiodothyronine and dexamethasone were also tested under similar conditions, but none of them caused a significant induction of lipogenic enzymes or lipogenesis. The secretion of very low-density lipoprotein (VLDL) triacylglycerol increased with culture time in the presence of oleate, whereas there was a large decline in the extracellular concentration of fatty acid. Thus, there was no simultaneous relationship between the extracellular fatty acid concentration and the secretion of VLDL. The effects of pancre-atic hormones and cyclic AMP on long-chain fatty acid metabolism were also investigated. Glucagon (10^-6 M) increased endogenous ketone body production by 150%. This resulted from alterations in the long-chain fatty acid due to esterification toward oxidation. We concluded from these results that insulin exerts long-term stimulation of lipogenesis by inducing enzymes related to lipogenesis, including G6PDH and malic enzymes, as well as causing slight stimulation by enhancing the supply of substrates for lipogenesis. These results are also the first direct evidence of ketone body production by glucagon in primary cultures of rat hepatocytes.

Key words: pancreatic hormone; Iipogenic enzymes; Iipogenesis; ketone body; hepatocytes

RETURN